Glaucophane schists, hornblende rocks, and eclogites are intimately associated within the Franciscan formation of the Healdsburg quadrangle. Discontinuity of megascopic structures and rapid variation in rock type indicate that the schists have undergone considerable disturbance since formation. However, weak trends conforming to the regional structure of the Franciscan formation can still be discerned in the metamorphic rocks. Most of the metamorphic rocks are derived from basic igneous rocks. The parents of the pumpellyite-Iawsonite-glaucophane schists are dense aphanitic volcanic rocks termed greenstones. The quartz-rich rocks show close chemical affinities with Franciscan cherts and arkosic wackes.
Eclogites bearing almandite garnet and acmitic diopside-jadeite are mineralogically atypical with reference to eclogites found elsewhere. It would seem that at one time they were more extensively developed in the area, for several existing rock types appear to have formed from them by retrograde processes. Retrograde products are members of the albite-epidote-amphibolite, greenschist, and glaucophane schist facies. Some members of the first two groups have been subsequently modified by the crystallization of glaucophane. Final products in such rocks are chlorite-glaucophane schists with remnant hornblende, and muscovite-chlorite-glaucophane schists with remnant pyroxene. Conditions accompanying the development of the eclogite are unknown.
Although serpentinite is associated with the group, the genesis of the schists and eclogite appears to be unrelated to it. The similarity in chemical composition between the schists and unaltered basaltic rocks and sediments of the formation suggests that the metamorphism was not accompanied by metasomatism.